The present invention relates to an improvement of the oxidation and decomposition resistance property of negative electrode materials for providing nonaqueous electrolyte secondary batteries having large capacity. More particularly, the present invention relates to nonaqueous electrolyte secondary batteries excellent in recovery property from overdischarge.
In recent years, lithium ion batteries using LiCoO2 as the positive electrode and a carbon material as the negative electrode have been widely used as power supplies for portable equipment. With advances in reduction in the size and weight of electronic equipment, these batteries have been requested to have higher energy density, and studies on various materials have been vigorously carried out.
As positive electrode materials for nonaqueous electrolyte secondary batteries, transition metal oxides and chalcogenide compounds such as LiMn2O4, LiFeO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2 and MoS2, have been proposed so far in addition to LiCoO2.
As negative electrode materials, on the other hand, various materials have been examined and carbon materials have been commercialized and have come into widespread use. However, carbon materials have already achieved a capacity near their theoretical capacity (about 370 mAh/g) and, therefore, a novel negative electrode material having a large capacity is desired.
Here, in JP-A-9-102311, there is proposed the use of a nitride represented by the formula: AxMyN, wherein A is at least one selected from the group consisting of Na, K, Mg, Ca, Ag, Cu, Zn and Al, M is a transition metal element (Mxe2x89xa0A), and 0.0 less than x and yxe2x89xa63.0 are met, as a support for a negative electrode active material in order to enhance the energy density of secondary batteries. In accordance with this publication, the element represented by A in the formula can be intercalated/deintercalated into/from the nitride, and moreover lithium ions can be intercalated/deintercalated.
However, the above material does not contain lithium at the time of synthesis thereof. Therefore, to intercalate/deintercalate lithium as a negative electrode active material of a lithium ion battery, it is necessary to deintercalate the element A in the above formula in some way to secure the site for lithium or directly intercalate lithium into a crystal lattice.
It is therefore considered more appropriate to use a composite nitride containing lithium, as proposed in JP-A-7-78609, from the viewpoint of easiness in production of a lithium ion battery, constructional stability during charge/discharge, and reversibility of charge/discharge. The lithium-containing composite nitride proposed in this JP-A-7-78609 is represented by the formula: LiaMbN, wherein M is a transition metal and a is a variable indicating the content of lithium in the active material varying with charge/discharge, and contains lithium from the beginning which is to be intercalated/deintercalated with charge/discharge. The average operating electrode potential is around 0.8 V with reference to the lithium potential. In addition, the reversible capacity of a battery using this material as the negative electrode is significantly large compared with the case of carbon materials. In other words, this is a promising material for attaining high-capacity batteries.
The present inventors evaluated a battery produced using LiCoO2 as the positive electrode material, in which lithium was electrochemically deintercalated in advance, and a lithium-containing composite nitride as the negative electrode material, which is represented by the formula: Li3xe2x88x92xMxN, wherein M is a transition metal and 0.2xe2x89xa6xxe2x89xa60.8. As a result, although no problem occurs under normal use and ambient conditions, it was found that the battery swells when the battery is in the overdischarged state and generates a gas, and the subsequent recovery is not sufficient. It is noted, herein, that the reaction of the negative electrode active material (that is, the lithium-containing composite nitride) absorbing (intercalating) lithium is referred to as xe2x80x9cchargexe2x80x9d, while the reaction thereof desorbing (deintercalating) lithium is referred to as xe2x80x9cdischargexe2x80x9d.
The mechanism of the gas generation in the overdischarged state is presumed as follows. The lithium-containing composite nitride immediately after the synthesis is in the state of being filled with lithium, unlike carbon materials. This state corresponds to the charged state of a battery using the lithium-containing composite nitride as the negative electrode. Therefore, the lithium-containing composite nitride is lower in stability and thus higher in reactivity when it is in the discharged state with lithium deintercalated than when it is in the initial charged state. This is presumably the reason for the gas generation.
The present inventors analyzed the generated gas and found that nitrogen was the main component. From this fact, it is presumed that this gas is not a gas generated by decomposition of an electrolyte but a gas caused by the lithium-containing composite nitride, that is, a gas generated by decomposition of the lithium-containing composite nitride.
The charge of the lithium-containing composite nitride during discharge is compensated by the valence change of the contained transition metal element as the lithium is being deintercalated from the lithium-containing composite nitride. In this way, presumably, the electric neutrality of the lithium-containing composite nitride is maintained.
However, it is expected that as the amount of lithium to be deintercalated increases, the charge will no more be compensated only by the valence change of the transition metal, and a load will be applied to the nitrogen element. For example, if the composition of the lithium-containing composite nitride is represented by the formula: Li2.6Co0.4N, the composition presumably changes to Li1.0Co0.4N when lithium is deintercalated.
On the assumption that, in the initial state of the compound (Li2.6Co0.4N), the oxidation numbers of lithium (Li), cobalt (Co) and nitrogen (N) are formally +1, +1 and xe2x88x923, respectively, and that Co is in charge of the entire charge compensation, the oxidation number of cobalt will become +5 in the discharged state (Li1.0Co0.4N). This oxidation number implies a high oxidation state normally inconceivable. In such a state, presumably, nitrogen also participates in the charge compensation by supplying electrons. Therefore, in the overdischarged state in which the nitride is further oxidized, it is presumed that the binding of the nitrogen in the lithium-containing composite nitride becomes very unstable, resulting in decomposition of the nitride.
An object of the present invention is to solve the above problem and to attain a highly reliable battery excellent in recovery property by avoiding the overdischarged state.
To attain the above object, the present invention provides a negative electrode material for a nonaqueous electrolyte secondary battery, comprising a lithium-containing composite nitride represented by the formula (1): Li3xe2x88x92xxe2x88x92yAyMxN wherein A is at least one selected from the group consisting of alkaline metals except for lithium and alkaline earth metals, M is a transition metal, 0.2xe2x89xa6xxe2x89xa60.8 and 0.0 less than yxe2x89xa60.8.
Preferably, the negative electrode material comprises core particles made of a lithium-containing composite nitride represented by the formula (2): Li3xe2x88x92xMxN, wherein M is a transition metal and 0.2xe2x89xa6xxe2x89xa60.8, and surface layers made of the lithium-containing composite nitride covering the respective core particles.
Further, the present invention provides a method for producing a negative electrode material for a nonaqueous electrolyte secondary battery, comprising the steps of:
(a1) synthesizing core particles made of a lithium-containing composite nitride represented by the formula (2): Li3xe2x88x92xMxN wherein M is a transition metal and 0.2xe2x89xa6xxe2x89xa60.8;
(b1) mixing the core particles with fine powder of a lithium-containing composite nitride represented by the formula (1): Li3xe2x88x92xxe2x88x92yAyMxN wherein A is at least one selected from the group consisting of alkaline metals except for lithium and alkaline earth metals, M is a transition metal, 0.2xe2x89xa6xxe2x89xa60.8 and 0.0 less than yxe2x89xa60.8, to give a mixture; and
(c1) applying mechanical energy to the mixture to form surface layers made of the lithium-containing composite nitride represented by the formula (1) on the surfaces of the respective core particles by mechanochemical reaction.
Still further, the present invention also provides a method for producing a negative electrode material for a nonaqueous electrolyte secondary battery, comprising the steps of:
(a2) synthesizing core particles made of a lithium-containing composite nitride represented by the formula (2): Li3xe2x88x92xMxN, wherein M is a transition metal and 0.2xe2x89xa6xxe2x89xa60.8;
(b2) mixing the core particles with at least one selected from the group consisting of alkaline metals except for lithium, alkaline earth metals and nitrides of these alkaline metals and alkaline earth metals to give a mixture; and
(c2) re-baking the mixture at 450xc2x0 C. or more to form surface layers made of the lithium-containing composite nitride represented by the formula (1): Li3xe2x88x92xxe2x88x92yAyMxN wherein A is at least one selected from the group consisting of alkaline metals except for lithium and alkaline earth metals, M is a transition metal, 0.2xe2x89xa6xxe2x89xa60.8 and 0.0 less than yxe2x89xa60.8 on the surfaces of the respective core particles.
The present invention also provides a nonaqueous electrolyte secondary battery using a negative electrode material comprising a lithium-containing composite nitride represented by the formula (1):
Li3xe2x88x92xxe2x88x92yAyMxN
wherein A is at least one selected from the group consisting of alkaline metals except for lithium and alkaline earth metals, M is a transition metal, 0.2xe2x89xa6xxe2x89xa60.8 and 0.0 less than yxe2x89xa60.8.
As the alkaline metal, sodium (Na) and potassium (K) are preferably exemplified and, as the alkaline earth metal, magnesium (Mg) and calcium (Ca) are preferably exemplified.
As the transition metal, at least one selected from the group consisting of Co, Cu, Ni, Fe and Mn can be exemplified.
As the nitride of the alkaline metal, preferred is at least one selected from the group consisting of Na3N and K3N. The present invention also provides a nonaqueous electrolyte secondary battery including a negative electrode using the above negative electrode material, a nonaqueous electrolyte, and a positive electrode capable of intercalating/deintercalating lithium.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.